The aerodynamic performance, hence efficiency, of an airfoil, such as a rotor blade, wing, turbine/compressor blade, or windmill blade, is strongly dependent on the ratio of the lift-to-drag (L/D ratio) forces generated by the airfoil. To this end, active flow control (AFC) techniques have been utilized to increase the L/D ratio of such airfoils. These AFC techniques include providing ports and/or openings through the surface of such airfoils and applying steady blowing, steady suction, or alternating blowing and suction of fluid therefrom. Such AFC techniques have proven to be effective in increasing the lift coefficient of an airfoil, decreasing the drag coefficient, or both in a manner increasing the overall L/D ratio of the airfoil, and thereby increasing the airfoil's aerodynamic efficiency.
AFC techniques are particularly advantageous in situations where large flow separations over airfoils would otherwise exist. Such situations are common on flapped airfoils during periods when relatively high lift is being generated. As is the case with the deployment of virtually all types of aerodynamic control surfaces, a drag penalty is usually incurred as a result of the deflection of a trailing edge flap system (be it a simple hinged plain flap or a more complex multiple-element slotted flap such as a Fowler flap). This drag penalty is a direct result of the creation of a local separated flow region whose size depends on the free stream angle of attack, the flow speed, the flap chord length, and the flap deflection angle. By reducing or delaying flow separation, a corresponding increase in lift and/or reduction in drag can be achieved.